1. Field
The present specification generally relates to wavelength-converted light sources and, more specifically, to methods for controlling wavelength-converted light sources to reduce speckle in projected images.
2. Technical Background
Laser speckle causes undesirable effects associated with the use of coherent illumination in general and laser-based projection systems in particular. Laser speckle is an undesirable random intensity variation perceived by the eye or other detection system. Speckle may result when a coherent light source is used to illuminate a rough surface such as a screen, wall, or any other object that produces a diffused reflection or transmission. Particularly, a multitude of small areas of the screen or other reflecting objects scatter light into a multitude of reflected beams with different points of origination and different propagation directions. At an observation point, such as the eyes of an observer or an optical detector such as the sensor of a camera, these reflected beams interfere constructively to form a bright spot or destructively to form a dark spot, thereby producing a random granular intensity pattern known as speckle.
The standard quantitative measure of the amount of speckle visible is speckle contrast. Speckle contrast is defined as the ratio between the standard deviation of intensity and the mean intensity detected by the eye or another optical detector. When the speckle contrast is 100% the speckle is considered to be “fully developed” which yields the lowest image quality to an observer and/or an optical detector. The human eye can detect speckle at contrast levels as low as 5% or even 1% depending on a number of variables including the wavelength of the projected light and brightness of the illumination as well as variations in perception amongst different individuals.
There is significant interest in image projectors based on coherent laser sources instead of incoherent lamps or LEDs. This is because of the potential compactness, power efficiency, brightness, color saturation and other properties afforded by laser sources. Because of the human eye's ability to detect speckle in a projected image, even modest amounts of speckle may be distracting to an observer and detract from the perceived quality of a projected image. A typical image projection system requires laser sources of three different colors which are typically red, green and blue. While red and blue laser sources are available, there is presently no practical laser source that emits at a green wavelength suitable for image projection systems. As a result, green laser sources used in projection systems generally comprise a pump laser coupled to a nonlinear wavelength conversion device which produces the desired green wavelength.
Accordingly, a need exists for alternative methods for controlling a wavelength-converted light source to reduce the perceived speckle contrast in an image projected with the wavelength-converted light source.